![]() Now you need to take this interesting octet and convert it from binary to decimal. We call the octet where the network address space stops the interesting octet. We know that the subnet is going to be the leftmost bit that are all 1s, so these will get added to the /16 network (Class B) bit giving you 11111111.11111111.11110000.00000000 (20 network bits / 12 address bits). ![]() ![]() We can do this by finding the classful mask that is not greater than the CIDR number. We need to first find the address bits that are going to get borrowed. Now with CIDR we are going to borrow bits from the classful subnet. A good strategy on test day will be to write this table out on your notepad all the way to 128 (or in binary 10000000). Now if we did 1+4 we would get 5 or 00000101. Every time that 1 move one place over to the left, the decimal value will increase by a power of 2. If you are not sure why 11111111 would get you 255 for these octets lets do a quick review of numeric to binary conversion.Īnd so on. 11111111.00000000 (the first 24 bitsĪre the network bits remaining 8 are address bits) So this means that the subnet bits (networks address bits) would beĬlass A - 11111111.00000000.00000000.00000000 (the first 8 bits are the network bits remaining 24 are address bits)ġ1111111. Once you get the hang of it though it's super easy. No better time to start then When I first encountered subnetting I had to read through a few approaches before it finally stuck with me. Subnetting may not be a huge part of the test, but it's definitely something that OP should learn if they plan on advancing in the field. Subnetting may also not be a huge part of the test. Since they give little dry erase sheets during exams, it's easy to quickly jot a chart down and reference it when the questions come up. If you have access to the Sybex book for the 70-410 exam, the explanation and charts are much better. Practice and check your results against this chart: Opens a new window 2^12 = 4096 which are enough addresses to cover the request. Working backwards (using the 0s) is easier since you can memorize the powers of 2. If you're asked to come up with a subnet mask with at least 4000 addresses, you can multiply 254 until you figure it out but that's not easy. You can have 4064 hosts with the /20 subnet mask. You can have 16 networks with that mask.Īddresses range from 0 to 255 for 256 total addresses but 0 and 255 are reserved leaving 254 addressable addresses. The last "1" in the mask represents the value "16". The heart of it is learning to calculate the binary values of the subnet mask. This may not be the easiest or most correct but it's the way I've been able to memorize subnetting:
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